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Periodontal disease affects cats that do not have good dental hygiene, Cornell University College of Veterinary Medicine reports. Although it may be difficult at first, owners should brush their cats' teeth regularly. Regular dental cleanings by veterinarians are also important, and these cleanings can reverse the early stages of periodontal disease. Food, string or other materials caught between the teeth cause bacterial infection and bad breath. Deformities or abnormalities in the mouth also cause bad breath.

Some bad breath is caused by larger health problems, according to WebMD. Diabetes often causes cats' breath to have a fruity or sweet odor. Liver disease and intestinal blockages also cause bad breath. Kidney disease may cause the cat's breath to smell like urine. These diseases generally come with other symptoms as well. Liver disease often comes with vomiting and lethargy, for example.

Other factors that cause bad breath include injuries or ulcers in the lips and gums, as reported by Cornell University College of Veterinary Medicine. Some cancers, particularly oral cancers, also contribute to foul-smelling breath. Most of the conditions that cause bad breath can be treated or cured by veterinarians, so cats suffering from these conditions should be seen by a veterinarian.